Design of Quad-Channel High-Temperature Superconducting Diplexer Using Spiral Stub-Loaded Resonators

A quad-channel high-temperature superconducting (HTS) diplexer with high isolation using spiral stub-loaded resonators (SSLRs) is presented in this paper. The quad-channel diplexer contains two paths with four SSLRs. Each path consists of two passbands that each passband shares two common SSLRs for size reduction. The SSLR structure of this study offers several design freedoms, such as the passband frequency can be adjusted by properly tuning its geometrical parameters and its bandwidth can be controlled through changing the coupling coefficients between the adjacent SSLRs. Moreover, a T-junction matching network is designed to minimize the undesired interactions between the two paths of the diplexer. To verify the designed methods, an HTS quad-channel diplexer operating at 1.91/2.157/2.36/2.59 GHz is successfully designed and fabricated by using YBCO thin film on a 0.5-mm-thick MgO substrate and it shows a compact size of 0.16λ
_g
× 0.23λ
_g
. The maximum insertion loss of four passbands is 0.28 dB and the isolation between channels at the four passbands higher than 50 dB is obtained by adopting T-junction matching network.